Within the frame of the production of polylactide (PLA) which is a biodegradable polymer, it becomes more and more important to be able to prepare copolymers with different contents of D-lactide.
While it is known that D-lactide may currently be obtained by fermentation of lactic acid of the D-configuration, it is also possible to recover D-lactide or meso-lactide starting from PLA after a depolymerization treatment of it.
It is therefore of importance to develop a new way, rather than, starting with lactic acid, to increasingly produce D-lactide and meso-lactide in order to prepare PDLA or copolymers containing both L and D-lactic acid enantiomer to form copolymers having different proportions of D and L units.
There is thus a need for such a process.
Normally and conveniently, the polymer or mixture of polymers subject to the depolymerization process comprises higher polymeric polylactide and is heated to produce cyclic esters. Such a process is generally very slow. On top of that those depolymerization processes are focused more on the purity of the lactide to recover than on a racemization to produce the other enantiomer.
WO2014/000277 discloses a method for using a recycled polylactic acid to prepare lactide. The method comprises a) crushing recycled polylactic acid, b) melting and extruding the crushed recycled polylactic acid and entering the melt into a pre-depolymerization reactor to obtain a polylactic acid melt, c) making said melt undergo a chain scission reaction at 180° C. to 250° C. with a catalyst to break the molecular chain of the melt (Mn<5000), d) making the polylactic acid melt with broken chains undergo a depolymerization reaction at 150° C. to 250° C. with the degree of vacuum being −0.1 MPa to −0.09 MPa, to produce crude lactide, e) separating the crude lactide through melt crystallization to obtain refined lactide. The content of the lactide can reach higher than 99.5% and the optical purity can reach higher than 99.9%.
The Chemical and Pharmaceutical bulletin, Pharmaceutical Society of Japan, vol. 47, no 4, 1 Apr. 1999, pages 467-471 discloses a series of Al, Ti, Zn and Zr compounds which were evaluated as intramolecular transesterification catalysts for the thermal depolymerization reaction of poly(L-lactic acid) oligomer resulting in LL-lactide, meso-lactide and DD-lactide.
EP 2 607 399 discloses a process for producing polylactic acid and reactors for use in said process. Example 2 refers to a depolymerization process of polylactic acid oligomer with tin oxide catalyst.
US 2013/0023674 provides a method for the production of lactide directly from recycled PLA wastes using thermal depolymerization process. Such depolymerization was exemplified in the presence of Ti (II) catalyst. The product recovered was mainly L-lactide.
Different patents, like Japanese Patent 2008-201679, also relate to the use of a catalytic system comprising in addition to the catalyst an acid compound, without other restriction, in order to improve depolymerization.
A need exists therefore for an improved process for depolymerizing polylactide, particularly and selectively to cyclic esters, i.e., D-lactide, meso-lactide and L-lactide.